1. Field of the Invention
The present invention relates to a technique for collating a target image as a subject of collation and inspection with a reference image as a sample of collation and inspecting the target image.
2. Description of the Related Art
An inspection method using collation of an image has been heretofore utilized in various kinds of inspection processes such as visual inspection of various kinds of objects, inspection of printed matter for image quality defects (such as whitening or staining). For collation of an image, a reference image is generally prepared as a sample of collation for indicating a normal state of the image. A target image which is a subject of collation is acquired. The target image and the reference image are superposed on each other and compared/collated with each other to thereby detect a mismatch portion (i.e. judge whether the two images are matched or not).
The target image acquired, however, contains geometrical distortion such as positional displacement or scaling up/down (change in magnification). There is a problem that highly accurate collation cannot be provided when the reference image is merely interposed on the target image.
The target image is studded with a large number of portions suffering from geometrical distortion such as positional displacement or scaling up/down. Measures against such geometrical distortion are known as follows. For example, Japanese Patent No. 3,140,838 has described a method in which: a plurality of split images are generated from at least one of a reference image and a target image as a subject of inspection; positional displacement correction values are calculated according to the split images; and positional displacement of the image is corrected on the basis of the correction values. For example, JP-A-11-194154 has described a method in which: a target image as a subjection of inspection is split into split images each having a size sufficient to ignore geometrical distortion; a reference image is split into split images in accordance with the size; positional displacement of each split image is detected with accuracy capable of measuring a size of not larger than a pixel size; and a judgment is made on the basis of the detected positional displacement as to whether the target image is defective or not. In the methods described in Japanese Patent No. 3,140,838 and JP-A-11-194154, geometrical distortion however remains in each generated split image if geometrical distortion is unevenly scattered in the target image. As a result, geometrical distortion remains in each block image. There is a problem that collation accuracy is lowered.
As is obvious from the techniques in the related art, geometrical distortion unevenly scattered in the target image is required to be entirely removed so that the target image can be pixel-by-pixel collated with the reference image accurately. As the related art for this requirement, there is known an inspection method, for example, as described in JP-A-2002-181732, in which: an image of an object provided for calibration and having a plurality of identification marks having known positional relations is photographed so that geometrical distortion components appearing in the image are examined and held as geometrical distortion calibration data in advance; and geometrical distortion of the target image is corrected on the basis of the geometrical distortion calibration data before image collation is performed. In the inspection method, because calibration data are acquired and held in advance, there is no problem if the trend of generation of geometrical distortion in the target image is unchanged at every time of collation/inspection. If the trend of generation of geometrical distortion is changed at every time, it is however conceived that the trend of geometrical distortion calibration data is different from the trend of actual geometrical distortion. There is a problem that collation accuracy is lowered because the influence of geometrical distortion cannot be entirely removed.
There is also known a method, for example, as described in JP-A-8-35936, in which: split images are generated from a reference image and a target image as a subjection of inspection; the split images of the target image are compared with the split images of the reference image respectively to thereby generate a plurality of distortion vectors indicating positional changes; and positional changes are corrected on the basis of the distortion vectors. In this method, the average trend of geometrical distortion scattered in the split images can be removed in the same manner as in the case where correction is performed in accordance with each block. It is however impossible to remove distortion disagreeing with the typical trend if geometrical distortion is unevenly scattered in the split images. As a result, there is a problem that collation accuracy is lowered because geometrical distortion remains in the split images.
There is further known a fingerprint checking apparatus, for example, as described in JP-A-7-57085, in which: waveforms of longest ridge width distributions in X and Y directions are obtained from a fingerprint to be registered; only characteristic peaks (limbs) are extracted from the waveforms of X- and Y-direction longest ridge width distributions in accordance with a predetermined rule; and waveforms of X- and Y-direction longest ridge width distributions formed from the limbs (i.e., longest ridge width local maximal value extraction waveforms) are stored as registered data to be used for positional correction. In this apparatus, it is however impossible to remove the influence of scaling up/down because there is no measure against scaling up/down of the target image. There is therefore a problem that collation accuracy is lowered.
As described above, the techniques in the related art have a problem that geometrical distortion, such as positional displacement or scaling up/down, unevenly scattered in the target image cannot be removed efficiently so that highly accurate inspection cannot be performed.